1. Field of the Invention
The invention relates to an unbalance vibration generator adapted to be driven at varying rotational speeds.
2. Description of the Prior Art
It is desirable to operate unbalance vibration generators at different rotational speeds, in order, for example, to be able to select the vibration frequency of a soil compactor in conformity with the elastic characteristics of a soil to be compacted, or in order to cause some other vibratory mechanical system to vibrate at its natural frequency. It has been found disadvantageous that the centrifugal force acting on a rotating, unbalanced unbalance mass varies with the square of the rotational speed. Therefore with a conventional unbalance vibration generator any variation of the rotational speed will automatically involve variation of the exciting force. This may have the result that either the exciting force becomes undesirably small, in the lower range of rotational speeds, or, if the unbalance is made correspondingly large, undesirably large exciting forces are obtained in the upper range of rotational speeds. Such large exciting forces, which are often not at all required, cause a correspondingly large load on the bearings. Therefore it is, in addition, desirable to provide a vibration generator in which the exciting force remains substantially constant with variations of the rotational speed within a certain range.
To this end a prior art unbalance vibration generator has two unbalance masses angularly offset by 180.degree.. One unbalance mass is affixed to a rotor shaft of the unbalance vibration generator. The second unbalance mass is radially movable relative to the first one and is subjected to the action of a spring which acts in radial direction and counteracts the centrifugal force acting in the second unbalance mass. Therefore the unbalance of the unbalance means is the larger and the resultant unbalance of both unbalance masses is the smaller the higher the rotational speed is. Due to the linearity of the spring characteristic, however, the exciting force is kept constant within a very coarse approximation only.